Updated, 7:45p.m.: CHP has announced that the roadway is now completely clear in both directions.
Updated, 1p.m.: The road is now open but one lane may be closed in the near future to allow tow truck access to the scene of the collision and so intermittent delays will continue. Please continue to drive cautiously in the area. The following article has been updated to include additional details about the collision.
MENDOCINO Co., 5/2/18 — A large car accident has closed Hwy. 20 in both directions just west of Potter Valley Rd. — that is, between Lake Mendocino and the road into Potter Valley — as of 12:20 p.m. The crash appears to be a small car vs. a big rig, with multiple injuries and air ambulances called.
The California Highway Patrol has stated that the road should be reopened in about half an hour.
According to the CHP incident log, the driver of the big rig is injured and there is major front end damage to the Prius, as well as “fluid leaking” out of the car. Debris covered the roadway as a result of the collision, and then there was additional difficulty in getting the tow truck to the scene due to the traffic caused by the accident — one truck was stuck in traffic on the way there.
Here’s the most recent screen shot of the CHP incident log, taken at 12:55p.m.:

CHP incident log of the accident.
Here are more details on the crash from our friends at Mendocino Sports Plus: